We describe observed properties of the Type Iax class of supernovae ( SNe Iax ) , consisting of SNe observationally similar to its prototypical member , SN 2002cx .
The class currently has 25 members , and we present optical photometry and/or optical spectroscopy for most of them .
SNe Iax are spectroscopically similar to SNe Ia , but have lower maximum-light velocities ( 2000 \lesssim|v| \lesssim 8000 km s ^ { -1 } ) , typically lower peak magnitudes ( -14.2 \geq M _ { V, { peak } } \gtrsim - 18.9 mag ) , and most have hot photospheres .
Relative to SNe Ia , SNe Iax have low luminosities for their light-curve shape .
There is a correlation between luminosity and light-curve shape , similar to that of SNe Ia , but offset from that of SNe Ia and with larger scatter .
Despite a host-galaxy morphology distribution that is highly skewed to late-type galaxies without any SNe Iax discovered in elliptical galaxies , there are several indications that the progenitor stars are white dwarfs ( WDs ) : evidence of C/O burning in their maximum-light spectra , low ( typically \sim 0.5 M _ { \sun } ) ejecta masses , strong Fe lines in their late-time spectra , a lack of X-ray detections , and deep limits on massive stars and star formation at the SN sites .
However , two SNe Iax show strong He lines in their spectra .
The progenitor system and explosion model that best fits all of the data is a binary system of a C/O WD that accretes matter from a He star and has a deflagration .
At least some of the time , this explosion will not disrupt the WD .
The small number of SNe in this class prohibit a detailed analysis of the homogeneity and heterogeneity of the entire class .
We estimate that in a given volume there are 31 ^ { +17 } _ { -13 } SNe Iax for every 100 SNe Ia , and for every 1 M _ { \sun } of iron generated by SNe Ia at z = 0 , SNe Iax generate \sim 0.036 M _ { \sun } .
Being the largest class of peculiar SNe , thousands of SNe Iax will be discovered by LSST .
Future detailed observations of SNe Iax should further our understanding of both their progenitor systems and explosions as well as those of SNe Ia .